Obtaining an aliphatic dialdehyde Monoacetal

ABSTRACT

The invention relates to a process for obtaining a pure aliphatic dialdehyde monoacetal by reaction of the corresponding aliphatic dialdehyde or a precursor of the corresponding aliphatic dialdehyde with one or more aliphatic mono- or polyhydric alcohols while distillatively removing water to obtain a reaction mixture which is separated distillatively, which comprises carrying out the distillative separation continuously in a dividing wall column to obtain pure aliphatic dialdehyde monoacetal as a sidestream from the dividing wall column, or in two distillation columns to obtain crude aliphatic dialdehyde monoacetal as a sidestream in the first distillation column, feed the crude aliphatic dialdehyde monoacetal to the second distillation column and obtain pure aliphatic dialdehyde monoacetal as the sidestream from the second distillation column.

CROSS REFERENCE TO RELATED APPLICATION

The present application is a National Stage application ofPCT/EP2003/013370, filed Nov. 27, 2003, which claims priority fromGerman Patent Application No. DE 102 55 674.4, filed Nov. 28, 2002.

The invention relates to a process for obtaining a pure aliphaticdialdehyde monoacetal.

Dialdehydes are valuable synthetic building blocks in organic synthesis,in particular of pharmaceuticals, agrochemicals and also other activeand effective ingredients, as a consequence of the reactivity and thevariety of reaction possibilities of the aldehyde functions. Particularinterest attaches to dialdehydes in which one of the two aldehydefunctions is masked, i.e. protected. It is thus possible in thesynthetic sequence to selectively and protectively react both functionalgroups by suitable reactions in each case.

A particularly simple, at the same time effective and also easilydetachable protecting group is the aldehyde acetal. Therefore, aliphaticdialdehydes in which one of the two aldehyde functions has beenacetalized with alcohols or thiols, i.e. dialdehyde monoacetals and alsotheir substitution products, in particular constitute interesting andvaluable intermediates in organic synthesis.

The review article of C. Botteghi and F. Soccolini in Synthesis 1985,pages 592 to 604 discloses various synthetic possibilities fordialdehyde monoacetals of the general formula

where n=1, 2 or 3.

However, the synthetic routes described are unsuitable for industrialscale use.

According to the current state of the art, especially for theparticularly interesting monoacetals of glutaraldehyde, i.e. compoundscorresponding to the above general formula where n=3, the only economicsynthetic route on the industrial scale is the direct reaction ofglutaraldehyde with the corresponding alcohol.

To this end, the following variants in particular are known:

In the process of JP 48-39416, glutaraldehyde is reacted directly underacid catalysis with ethylene glycol in a 2:1 ratio. The process affordsthe product of value, the monoethylene glycol acetal of glutaraldehyde,2-(3-formylpropyl)-1,3-dioxolane (abbreviated to FPDO hereinbelow), in a40% yield after distillation. However, the excess of glutaraldehyde hasto be distillatively removed.

In the process of JP 48-61477, glutaraldehyde is reacted with an excessof ethylene glycol to give the diacetal. This is then hydrolyzed to givethe monoacetal after isolation with water. After extractivepurification, the product of value FPDO is obtained in a 48% yield.

In the process of JP 11-228566, glutaraldehyde is initially reacted withethylene glycol, likewise to give the diacetal. However, this thendisproportionates with further glutaraldehyde after isolation to givethe product of value FPDO.

The existing processes have in particular the following disadvantages:In all reactions, a mixture of reactant, the monoacetal product of valueand bisacetal is formed. In the process of JP 48-61477 or JP 11-228566in which the bisacetal is deliberately prepared initially, theequilibrium with regard to the products is more advantageous. However,as before, the mixture has to be separated; additionally, an additionalprocess stage is required.

One problem of all existing processes which has not yet been solved inan industrially and economically viable manner is that, as a consequenceof the high reactivity of the two aldehyde functions in the dialdehydereaction which is partially masked in the dialdehyde monoacetal product,the reaction mixtures, in particular at elevated temperature, readilypolymerize. This delivers highly viscous products which are difficult tohandle and lead to yield losses. Especially in the case of distillativeworkup as a batch distillation with high residence times at hightemperature, as carried out in the above-described processes, this leadsto product losses.

DETAILED DESCRIPTION OF THE INVENTION

It is an object of the invention to provide a process which leads in aneconomically advantageous manner in one stage to the dialdehydemonoacetal product of value in a high degree of purity of at least 98%by weight, in order to fulfill the specification requirements for use insubsequent syntheses, and in which the product losses by polymerizationare kept low. Especially in the distillation of the crude material,product losses should be minimized.

We have found that this object is achieved by a process for obtaining apure aliphatic dialdehyde monoacetal by reaction of the correspondingaliphatic dialdehyde or a precursor of the corresponding aliphaticdialdehyde with one or more aliphatic mono- or polyhydric alcohols whiledistillatively removing water to obtain a reaction mixture which isseparated distillatively, which comprises carrying out the distillativeseparation continuously in a dividing wall column to obtain purealiphatic dialdehyde monoacetal as a sidestream from the dividing wallcolumn, or in two distillation columns to obtain crude aliphaticdialdehyde monoacetal as a sidestream in the first distillation column,feed the crude aliphatic dialdehyde monoacetal to the seconddistillation column and obtain pure aliphatic dialdehyde monoacetal asthe sidestream from the second distillation column.

In the present context, the crude aliphatic dialdehyde monoacetal is amixture which is formed of at least 90% by weight, preferably of atleast 97% by weight, of the product of value, the aliphatic dialdehydemonoacetal.

In the present context, the pure aliphatic dialdehyde monoacetal is amixture which is formed of at least 98% by weight, preferably of atleast 99% by weight, of the product of value, the aliphatic dialdehydemonoacetal.

The invention is not restricted with regard to the specific performanceof the reaction of the aliphatic dialdehyde or of a precursor of thealiphatic dialdehyde with one or more aliphatic, mono- or polyhydricalcohols.

In a preferred variant, the dialdehyde, preferably glutaraldehyde, isinitially charged in aqueous solution, preferably up to 50% by weight inwater, and preheated to from 30 to 80° C., preferably from 40 to 50° C.,more preferably to 45° C. Subsequently, reduced pressure is applied sothat the water of solution distills off. At the same time as the wateris distilled off, alcohol or a mixture of alcohols, preferably ethyleneglycol, is added. Toward the end of the reaction, the temperature isincreased to from 50 to 110° C., preferably from 80 to 90° C., morepreferably to 85° C.

In a further process variant, the aliphatic dialdehyde, preferablyglutaraldehyde, which is present as an aqueous solution, is dewatered byapplying reduced pressure, preferably at slightly elevated temperature,in the range from 30 to 80° C., preferably from 40 to 50° C., morepreferably at 45° C. However, as a consequence of the tendency tospontaneous polymerization, care has to be taken that the dewatereddialdehyde is kept at a temperature within the abovementioned range andalso constantly in motion, and is reacted immediately after thedewatering. To this end, in a similar manner to the process variant, thealcohol or the mixture of alcohols, preferably ethylene glycol, isadded.

In a further process variant, it is possible to initially charge bothreactants, the dialdehyde and also the alcohol or alcohols andoptionally the catalyst, preferably using the dialdehyde in aqueoussolution and subsequently distilling off both the water of solution andthe water of reaction. However, the space-time yield in this processvariant is worsened compared to the above-described variants.

The aliphatic dialdehyde used is preferably a substance from thefollowing list: malonaldehyde, succinaldehyde, glutaraldehyde oradipaldehyde or their alkyl-substituted derivatives, more preferablyglutaraldehyde, in particular in aqueous solution, preferably in 50%aqueous solution, or its precursor 2-hydroxy-3,4-dihydo-2H-pyran.

The alcohol component used may in particular be a monohydric alcoholsuch as methanol, ethanol, n-propanol, i-propanol, n-butanol,sec-butanol, i-butanol, or a diol, in particular ethylene glycol,1,2-propylene glycol, 1,3-propylene glycol, 1,2-butanediol,1,3-butanediol or 1,4-butanediol, and particular preference is given toethylene glycol.

Particular preference is given to using glutaraldehyde with ethyleneglycol in a molar ratio in the range from 1:1.5 to 1.5:1, preferablyfrom 1:1.2 to 1.2:1, in particular of 1.0:1.0. Although the conversionto the aliphatic dialdehyde monoacetal also proceeds uncatalyzed,preference is given to using an acidic catalyst, in particular a cationexchanger, a mineral acid, preferably sulfuric acid, hydrochloric acid,more preferably orthophosphoric acid or an organic acid, in particularacetic acid, p-toluenesulfonic acid or methanesulfonic acid, in aconcentration of from 0.02 to 5% by weight, preferably from 0.1 to 1% byweight, more preferably of 0.3% by weight, based on the total weight ofthe reaction mixture.

The reaction mixture which has been virtually completely freed of thewater burden by distillation is subsequently distillatively separated toobtain the product of value, the aliphatic dialdehyde monoacetal.

The inventors have recognised that it is essential for this purpose tocarry out the distillation continuously. Compared to the existingdistillations carried out batchwise, continuous distillations have theadvantage of a shorter residence time of the liquid phase product andtherefore lower thermal stress and damage. By carrying out thedistillation continuously in accordance with the invention, asignificant improvement in the distillation yield is achieved.

The continuous distillative separation can be carried out in twosuccessive distillation columns or, particularly advantageously, in adividing wall column.

To perform the distillation in two successive distillation columns, thevirtually anhydrous reaction mixture is firstly fed to a firstdistillation column which advantageously has from 40 to 80 theoreticalplates, preferably from 50 to 70 theoretical plates, more preferablyfrom 60 to 70 theoretical plates, and continuously distilled at a toppressure of from 5 to 500 mbar, preferably from 10 to 300 mbar, morepreferably from 15 to 100 mbar.

Unconverted glutaraldehyde is removed as the top product and preferablyrecycled into the synthesis. Crude aliphatic dialdehyde monoacetal, i.e.a mixture which contains at least 90% by weight, preferably at least 97%by weight, of the monoacetal product of value, is removed from therectifying section of the column, i.e. above the feed of the mixture tobe separated.

At the bottom of the column, the diacetal and also more highly condensedproducts are obtained. Suitable bottom evaporators are in particularfalling-film evaporators, since they guarantee gentle evaporation andthus do not stress the thermally sensitive product.

Preference is given to separating the bottom effluent of the firstdistillation column in a downstream thin film evaporator into twostreams at a pressure of preferably about 10 mbar: the volatile diacetalis removed overhead, condensed and recycled to the acetalization stagefor dissociation. The high-boiling polymers are utilized thermally.

The crude aliphatic dialdehyde monoacetal is subsequently fed to asecond distillation column which preferably has from 30 to 70theoretical plates, in particular from 40 to 70 theoretical plates, morepreferably from 50 to 70 theoretical plates, and is operated at a toppressure of from 5 to 500 mbar, preferably from 10 to 300 mbar, morepreferably from 15 to 100 mbar.

Remaining dialdehyde is removed from the second distillation column as atop stream and preferably recycled into the synthesis.

Pure aliphatic dialdehyde monoacetal, i.e. a mixture which contains atleast 98% by weight of the dialdehyde monoacetal product of value,preferably 99% by weight of the product of value, is removed as avaporous sidestream from the stripping section of the column, i.e. belowthe feed of the mixture to be separated into the second distillationcolumn.

At the bottom of the column, higher-boiling components are obtainedwhich still contain fractions of the aliphatic dialdehyde monoacetalproduct of value. In order to reduce loss of product of value,preference is given to recycling the bottom stream into the firstdistillation column.

In a particularly advantageous process variant, the continuousdistillation is carried out in a single apparatus, a dividing wallcolumn.

It is known that dividing wall columns enable a particularly economicalseparation, which is advantageous especially with regard to the capitaland energy costs, of multicomponent mixtures to obtain one or more puresidestreams. In sections of a dividing wall column, transverse mixing ofthe liquid and vapor streams is prevented by dividing wall, generally ametal sheet disposed in the longitudinal direction of the column.Customarily, the dividing wall divides the column interior into a feedsection, a takeoff section, an upper combined column region and also alower combined column region. Between the feed region and the takeoffregion is disposed the dividing wall which prevents transverse mixing ofliquid and vapor streams over the entire column cross section in thesecolumn regions. This makes it possible to obtain a product in pure format a sidestream takeoff. The dividing wall may be welded fast or elseonly inserted loosely, the latter variant having the advantage of lowcapital costs.

To perform the distillative separation of the reaction mixture in thepresent process in a dividing wall column, the virtually anhydrousreaction mixture is fed to a dividing wall column which has a liquidsidestream and preferably from 40 to 100 theoretical plates, inparticular from 50 to 90 theoretical plates, more preferably from 60 to85 theoretical plates, and is operated at a top pressure of from 5 to500 mbar, preferably from 10 to 300 mbar, more preferably from 15 to 100mbar. In the dividing wall column, the reaction mixture is continuouslyseparated distillatively into three fractions: into a low boilerfraction which contains unconverted reactants and which is preferablyrecycled into the reaction stage, into a medium boiler fraction whichcontains the pure dialdehyde monoacetal, i.e. a mixture having a productof value of at least 98% by weight, preferably at least 99% by weight,and also a high boiler fraction which contains the diacetal and alsohigher-boiling components.

Particularly suitable bottom evaporators for the dividing wall columnare falling-film evaporators, since they ensure gentle evaporation anddo not stress the thermally sensitive product.

With regard to the separating internals, there is in principle norestriction, i.e. it is possible to use trays, random packings orstructured packings, for example sheet metal or fabric packings,preferably having specific surface areas of from 250 to 750 m²/m³.Particular preference is given to fabric packings as a consequence oftheir relatively low pressure drop per plate, and also their betterseparating performance in vacuum distillations, as used in the presentcontext.

The bottom effluent of the dividing wall column is subsequentlypreferably fed to a thin-film evaporator and separated there, preferablyat a pressure of about 10 mbar, into two streams: the volatile diacetalis removed in vaporous form, condensed and recycled into theacetalization stage for dissociation. The high-boiling polymers areutilized thermally.

The dividing wall column is preferably divided in such a way that allcolumn regions, i.e. the upper combined column region, the rectifyingsection of the feed region, the stripping section of the feed region,the rectifying section of the takeoff region, the stripping section ofthe takeoff region and also the lower combined column region each have 5to 50%, preferably from 15 to 30%, of the total number of theoreticalplates of the dividing wall column.

Preference is further given to the dividing wall column being designedin such a way that the sum of the number of theoretical plates of thetwo parts of the feed region, i.e. the rectifying section and thestripping section, is from 80 to 110%, preferably from 90 to 100%, ofthe sum of the number of theoretical plates of the two parts (rectifyingsection and stripping section) of the takeoff region.

Feed and sidestream takeoff can be disposed at different heights in thedividing wall column, and the feed is disposed preferably from 1 to 8theoretical plates, more preferably from 3 to 5 theoretical plates,higher or lower than the sidestream takeoff.

The division ratio of the liquid at the upper end of the dividing wallis preferably adjusted in such a way that the concentration of thosecomponents of the high boiler fraction for which a certain limitingvalue in the sidestream is predefined in the liquid at the upper end ofthe dividing wall is from 10 to 80%, preferably from 30 to 50%, of thevalue which is predefined for the sidestream product. At a highercontent of high boilers, the liquid division is adjusted in such a waythat more liquid is conducted to the feed region, while, at a lowerconcentration of high boilers, less liquid is conducted to the feedregion.

The heating output in the bottom evaporator is preferably adjusted insuch a way that the concentration of those components of the low boilerfraction for which a certain limiting value in the sidestream ispredefined at the lower end of the dividing wall is adjusted in such away that it is from 10 to 80%, preferably from 30 to 50%, of the valuewhich is predefined for the sidestream product. At a higher content ofcomponents of the low boiler fraction, the heating output is increased,and at a lower content, it is reduced.

The distillate is removed under temperature control, and the controltemperature used is a measuring point in the upper combined columnregion, which is disposed from 3 to 8, preferably from 4 to 6,theoretical plates below the upper end of the column.

The bottom product is likewise removed under temperature control. Thecontrol temperature is a measuring point in the lower combined columnregion which is disposed from 3 to 8, preferably from 4 to 6,theoretical plates above the lower end of the column.

The side product is preferably withdrawn under level control, and theliquid level in the bottom evaporator serves as the control parameter.

A cost comparison between the two variants of the distillativeseparation in two distillation columns connected in series on the onehand and in a dividing wall column on the other hand shows that thedividing wall column is about 30% cheaper both with regard to thecapital costs and the energy costs. A further advantage of separation ina dividing wall column is the distinct reduction in the thermal stresson the sensitive product, which results from the shortening of theresidence time in the bottom evaporator, especially as a consequence ofthe reduction to a single bottom evaporator.

In a particularly advantageous process variant, the substantiallyanhydrous reaction mixture is heated to from 80 to 130° C. before it isfed to distillative separation.

It has been found that, surprisingly, the viscosity of the reactionmixture can be significantly reduced by heating, especially into a rangewithin which it can be readily transported by pumps. In addition, theheating achieves a significant rise in product of value, the aliphaticdialdehyde monoacetal, in the reaction mixture.

According to the invention, the heating is effected at temperatures inthe range from 80 to 130° C., preferably from 90 to 110° C. The heatingtime is uncritical: a minimum duration of 15 minutes may be sufficient,and an upper limit is not decisive for the success of the invention, butrather at most conceivable on the basis of economic considerations.Preference is given to heating for from 30 minutes to 4 hours, morepreferably for 1 hour.

The pressure at which heating is effected is not critical:

Heating may be effected under reduced pressure, under increased pressureor at atmospheric pressure, but preferably at atmospheric pressure.

In a further particularly advantageous process variant, the distillativeseparation of the optionally heated reaction mixture is carried out withthe addition of a high-boiling diluent into the lower region of thefirst distillation column or into the lower combined column region ofthe dividing wall column.

The high-boiling diluent has to be miscible with the reaction mixture,it must not react with the reaction mixture and should have a lowervapor pressure than any individual component of the reaction mixture andalso than the reaction mixture. Preference is given to adding thehigh-boiling diluents in a proportion of from 1 to 30% by weight,preferably from 2 to 20% by weight, more preferably from 5 to 15% byweight, based on the mixture to be separated distillatively.

A particularly suitable diluent is a substance or a mixture ofsubstances selected from the following listed groups: alkanes, aromaticsor polyethers, preferably polypropylene glycols or polyethylene glycols,more preferably polyethylene glycol having an average molecular mass of300.

The addition of the high-boiling diluent prevents caking andpolymerization of the distillation bottoms to the heat exchangesurfaces, and thus improves the yields of product of value.

The invention is illustrated by the examples which follow and also adrawing.

EXAMPLES 1 TO 3 Reaction of glutaraldehyde with ethylene glycol to give2-(3-formylpropyl)-1,3-dioxolane (FPDO) Example 1 SimultaneousDistilling Off of Water of Solution and Addition of Ethylene glycol

A 1l stirred apparatus with an attached 10 cm randomly packed column ofRaschig rings and a distillation head with condenser was initiallycharged with 800 g of a glutaraldehyde solution (50% in water). At aninternal temperature of from 60 to 65° C. and a vacuum of 200 mbar, thewater was distilled off. As soon as the first distillate had beenobtained, a solution of 1.2 g of orthophosphoric acid (99%) in 248 g ofethylene glycol was added dropwise within two hours at the same time asthe water was distilled off. The reaction mixture was conducted at 65°C./200 mbar for a further hour after the addition. Afterwards, thevacuum was improved stepwise to 25 mbar, the internal temperature wasincreased to 85° C. and all of the water was distilled off. 530 g of avery viscous, colorless crude solution were obtained. Composition (GCarea %): 50.5% of FPDO, 36.9% of 1,3-bis(1,3-dioxolan-2-yl)propane(bisacetal), 9.0% of glutaraldehyde.

Example 2 Distilling Off Water of Solution Followed by Addition ofEthylene Glycol

A 2 l stirred apparatus having an attached 10 cm randomly packed columnof Raschig rings and a distillation head with condenser was initiallycharged with 1200 g of a glutaraldehyde solution (50% in water) andafterwards the water was distilled off at an internal temperature offrom 70 to 80° C. and a vacuum of 150 mbar. Subsequently, a solution of2 g of orthophosphoric acid (99%) in 372 g of ethylene glycol was addeddropwise at an internal temperature of from 75 to 83° C. within 90minutes and the reaction mixture was subsequently stirred for a further90 minutes. Afterwards, vacuum was applied which was improved stepwisefrom 100 to 50 mbar to distill off the water of reaction at an internaltemperature of from 70 to 88° C. 850 g of a very viscous, colorlesscrude solution was obtained. Composition (GC area %): 50.7% of FPDO,25.0% of 1,3-bsis(1,3-dioxolan-2-yl)propane (bisacetal), 12.7% ofglutaraldehyde, 3.0% of ethylene glycol.

Example 3 Distilling Off Water of Solution and Water of Reaction

A 1 l stirred apparatus with an attached 10 cm randomly packed column ofRaschig rings and a distillation head with condenser was initiallycharged with 800 g of a glutaraldehyde solution (50% in water), 248 g ofethylene glycol and 1.2 g of orthophosphoric acid (99%). The reactionmixture was stirred at 60° C. for 45 minutes. Afterwards, water wasdistilled off at 180 mbar within 3 hours. Subsequently, the vacuum wasimproved stepwise to 30 mbar and the internal temperature increased to90° C., in order to distill off all of the water. 568 g of a veryviscous, slightly cloudy crude solution were obtained. Composition (GCarea %): 56.4% of FPDO, 19.5% of 1,3-bis(1,3-dioxolan-2-yl)propane(bisacetal), 14.8% of glutaraldehyde, 2.2% of ethylene glycol.

Comparative Example Heating

A crude solution prepared according to example 1 having an FPDO contentdetermined by gas chromatography with an internal standard of 50% byweight was heated at 60° C. and atmospheric pressure under protectivegas. The FPDO content fell to 36% by weight of FPDO after heating for 24hours and to 29.4% by weight of FPDO after heating for 72 hours.

Example 4 Heating

Various samples of a crude solution prepared according to example 1which had been stored at 60° C. for a short time and whose FPDO contentdetermined by gas chromatography with an internal standard was 35.3% byweight were heated with variation of temperature and time. The FPDO(product of value) content and also the kinematic viscosities to DIN51562 were determined for the heated solutions.

The results are listed in the table 1 below:

Temperature Time [h] % by weight of FPDO 4.0 0 35.3 4.1  90° C. 1 40.04.2 3 41.4 4.3 5 41.7 4.4 100° C. 1 43.4 4.5 3 43.2 4.6 5 41.7 4.7 110°C. 1.5 45.6 4.8 3 46.6 4.9 120° C. 1.5 47.5 4.10 3 46.0

The kinematic viscosity of the crude solution of comparative example4.0, i.e. the unheated sample, was 6040 mm²/s at 20° C., whereas theviscosity of sample 4.8 (heated at 110° C. for 3 hours) was only 17.5mm²/s at 20° C. The heating therefore leads to a significant reductionin viscosity. In addition, the FPDO (product of value) content clearlyincreases, as can be seen in the last column of the above table.

Comparative Example Distillation

750 g of a crude solution prepared according to example 2 and having anFPDO content of 50% by weight were distilled batchwise in a 60 cmrandomly packed column of Raschig rings at a bottom temperature of 150°C., a vacuum of 1.5 mbar and a residence time in the bottom of about 10hours. In total, only 230 g of FPDO could be removed distillatively,which corresponds to a distillation yield of only 60%. The bottoms wereextremely viscous and polymerized solids which had formed had blockedthe lower column region.

Example 5 Simulation of the Thermal Stress on the Mixture in a Columnwithout addition of high-boiling diluent

To simulate the thermal stress on the crude solution in the distillationin a column equipped with a falling-film circulation evaporator, 500 gof a crude solution prepared according to example 1 and heated at 95° C.for 2 hours was continuously distilled at a feed rate of 500 g/h and aresidence time of approx. 1 minute at a temperature of from 150 to 155°C. and a vacuum to 2 mbar. After half of the feed, significant blackdeposits could be observed and the bottom outlet became blocked bypolymer, so that the experiment had to be terminated.

Example 6 Simulation of the Thermal Stress on the Mixture in a Columnwith the addition of high-boiling diluents

To simulate the thermal stress on the crude solution in the distillationin a column equipped with a falling-film circulation evaporator, 3700 gof crude material prepared according to example 1 and heated at 95° C.for 2 hours, except in a mixture with 10% by weight of polyethyleneglycol of molar mass 300, was continuously distilled on a thin-filmevaporator at a temperature of 135° C. and a vacuum of 1 mbar. The feedrate, as in example 5, was 500 g/h and the residence time about 1minute. The experiment was terminated after 7 hours, without any deposithaving been observed on the apparatus. 996 g of distillate and 1670 g ofbottom effluent were obtained.

Example 7 Distillation without Addition of High-Boiling Diluents

In a column (diameter 300 mm, Sulzer structured packing, 60 theoreticalplates), 10 t of crude material prepared in a similar manner to example1 and having an FPDO content of 40% by weight were continuouslydistilled in two stages. In the first stage (20 mbar; residence time:approx. 4 h), the monoacetal, FPDO, was initially obtained in a purityof approx. 95% in the liquid sidestream. In a second stage (15 mbar),the pure FPDO product was then likewise obtained in a vaporoussidestream. Unconverted glutaraldehyde and ethylene glycol were eachobtained overhead; the bisacetal was obtained via the bottom of thefirst distillation stage and was recycled back into the synthesis. 3.8 tof FPDO in a purity of >99% were obtained. The distillation yield was95%.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a distillation scheme having two distillation columnsconnected in series,

FIG. 2 shows a distillation scheme having a dividing wall column and

FIG. 3 shows a distillation scheme having a dividing wall column withthe inclusion of the control apparatus.

FIG. 1 shows a first distillation column K1 to which the anhydrousreaction mixture (stream A) is fed in the middle region. Thedistillation column K1 is equipped with a bottom evaporator and also acondenser at the top of the column. The top stream is condensed in thecondenser at the top of the column, partly removed as stream B1 whichcontains predominantly glutaraldehyde and the remainder is fed back tothe column as reflux. Crude FPDO (stream C1) is removed as a liquidsidestream from the rectifying section of the column. The bottom streamD is divided into two streams in a thin-film evaporator V, a top streamcontaining the volatile diacetal which is partly recycled to thesynthesis as stream E and a bottom stream comprising high boilers whichis discharged.

The crude FPDO (stream C1) is fed to the second distillation column K2in the middle region thereof. The column K2 is likewise equipped with acondenser at the top of the column and also with a bottom evaporator.The top stream of column K2 is condensed in the condenser at the top ofthe column, partly removed as stream B2 which consists predominantly ofglutaraldehyde, and the remainder is fed back to the column as reflux. Apure FPDO-containing stream (stream C2) is removed in vaporous form fromthe stripping section of column K2 and condensed. The bottom stream isrecycled into column K1.

FIG. 2 shows a dividing wall column K3 having a dividing wall TWdisposed in the longitudinal direction of the column and separating thecolumn interior into a feed region having a rectifying section 2 andstripping section 4, and also into a takeoff region having a rectifyingsection 3 and stripping section 5, and also into an upper combinedcolumn region 1 and a lower combined column region 6. The anhydrousreaction mixture is fed to the dividing wall column as stream A into themiddle region of the feed region, the top stream is condensed in acondenser at the top of the column, partly removed as stream Bcomprising predominantly glutaraldehyde and the remainder is fed back tothe column as a reflux stream. FPDO (stream C) is removed from thetakeoff region of the column. The bottom stream D is separated in athin-film evaporator V into a top stream comprising predominantly thediacetal which is recycled into the synthesis as stream I and also intoa bottom stream which is discharged.

The schematic representation in FIG. 3 illustrates the control apparatusfor the dividing wall column K3. TC indicates temperature controllers,LC is a liquid level controller and PDC is a differential pressuremeter.

1. A process for obtaining a pure aliphatic dialdehyde monoacetalcomprising a reaction of the corresponding aliphatic dialdehyde or aprecursor of the corresponding aliphatic dialdehyde with one or morealiphatic mono- or polyhydric alcohols while distillatively removingwater to obtain a reaction mixture which is separated distillatively,said process further comprising carrying out the distillative separationcontinuously in (i) a dividing wall column to obtain pure aliphaticdialdehyde monoacetal as a sidestream from the dividing wall column, or(ii) in two distillation columns to obtain crude aliphatic dialdehydemonoacetal as a sidestream in the first distillation column, feeding thecrude aliphatic dialdehyde monoacetal to the second distillation column,and obtaining pure aliphatic dialdehyde monoacetal as the sidestreamfrom the second distillation column.
 2. A process as claimed in claim 1,wherein the reaction mixture is heated to from 80 to 130° C. before thedistillative separation.
 3. A process as claimed in claim 1, wherein thereaction mixture is heated for at least 15 minutes.
 4. A process asclaimed in claim 1, wherein the aliphatic dialdehyde is selected fromthe group consisting of malonaldehyde, succinaldehyde, glutaraldehyde,and adipaldehyde.
 5. A process as claimed in claim 1, wherein thealiphatic dialdehyde is glutaraldehyde or its precursor,2-hydroxy-3,4-dihydro-2H-pyran.
 6. A process as claimed in claim 1,wherein the aliphatic mono- or polyhydric alcohol is a diol.
 7. Aprocess as claimed in claim 5, wherein glutaraldehyde is reacted withethylene glycol in a molar ratio in the range from 1:1.5 to 1.5:1.
 8. Aprocess as claimed in claim 1, wherein the reaction is carried out inthe presence of an acidic catalyst, in a concentration of from 0.02 to5% by weight based on the total weight of the reaction mixture.
 9. Aprocess as claimed in claim 1, wherein the optionally heated reactionmixture is continuously separated in two distillation columns to removethe crude aliphatic dialdehyde monoacetal as a sidestream in a firstdistillation column and the pure aliphatic dialdehyde monoacetal as asidestream in a second distillation column.
 10. A process as claimed inclaim 1, wherein the optionally heated reaction mixture is separated ina dividing wall column having a vertical dividing wall which is disposedin the longitudinal direction of the column and divides the column intoa feed region, a takeoff region, a lower combined column region and alsoan upper combined column region, to recover pure aliphatic dialdehydemonoacetal as a sidestream from the withdrawal region.
 11. A process asclaimed in claim 1, wherein the distillative separation of theoptionally heated reaction mixture is carried out with the addition of ahigh-boiling diluent in the lower region of the first distillationcolumn or in the upper combined column region of the dividing wallcolumn.
 12. A process as claimed in claim 11, wherein the high-boilingdiluent is a substance or a mixture of substances selected from thegroup consisting of: alkanes, aromatics or polyethers, preferablypolypropylene glycols, and polyethylene glycols.
 13. A process asclaimed in claim 3, wherein the reaction mixture is heated from 30minutes to 4 hours, at from 90 to 110° C.
 14. A process as claimed inclaim 13, wherein the reaction mixture is heated for 1 hour.
 15. Aprocess as claimed in claim 5, wherein the glutaraldehyde is used inaqueous solution.
 16. A process as claimed in claim 15, wherein theaqueous solution of glutaraldehyde is a 50% by weight aqueous solution.17. A process as claimed in claim 6, wherein the aliphatic diol isselected from the group consisting of ethylene glycol, 1,2-propyleneglycol, 1,3-propylene glycol, 1,2-butanediol, 1,3-butanediol, and1,4-butanediol.
 18. A process as claimed in claim 17, wherein thealiphatic diol is ethylene glycol.
 19. A process as claimed in claim 7,wherein glutaraldehyde is reacted with ethylene glycol in a molar ratioin the range from 1:1.2 to 1.2:1.
 20. A process as claimed in claim 19,wherein glutaraldehyde is reacted with ethylene glycol in a molar ratioin the range from 1.0:1.0.
 21. A process as claimed in claim 8, whereinthe acidic catalyst is selected from the group consisting of a cationexchanger, a mineral acid, and an organic acid.
 22. A process as claimedin claim 12, wherein the polyethylene glycol has an average molecularmass of 300.